Tornado Forecasting Notes
Loaded Gun Chasing
website of storm chaser Mikey Gribble
0-1km Helicity - 0-1km SRH discriminates more strongly than 0-3km helicity between significant
tornadic and nontornadic supercells. (Thompson)
Average 0-1km SRH for significant tornadic supercells in this study (28 tornado cases) was 140m2s2.
Moisture and 0-1km SRH - 77% of tornadic supercells (study of 28 tornado cases) occurred with
0-1km RH >65% and 0-1km SRH > 75m2s2, while 72% of the nontornadic supercells occurred with
lesser values of either paramater. So low-level shear and low-level moisture, in combination, can
strongly discriminate between significant tornadic and nontornadic supercells. (Thompson)
0-1km Shear - 0-1km vector shear magnitude > 15kts favors tornadic storms (Thompson)
5km SR Winds - threshold for tornadic supercells on 5km (500mb) SR winds is around 15kts
0-3km Helicity - Results of 28 tornado cases with the following categories of 0-3km helicity
150 < H < 299 = weak tornadoes
300 < H < 449 = strong tornadoes
H > 450 = violent tornadoes
Nonmesocyclone Tornadoes (Jon Davies)
(List of things to watch for in nonmesocyclone tornado environments based on Davies case study)
-Northeast to Southwest oriented front
- LCL heights are insignificant (sometimes > 2000m AGL)
- deep moist layer resulting in 0-3km MLCAPE of 90,40
- steep low-level lapse rates (near dry adiabatic) 0-2km
- watch for boundary intersections creating a conditional triple point or any other area where low-levlel
convergence and pre-existing vertical vorticity are focused
700 - 800mb Humidity (Doswell)
- 700-800mb humidity is important when forecasting tornadoes. By lowering the relative humidity,
stronger downdrafts are created. Lowering the relative humidity enough causes an outflow dominant
storm with little low-level rotation.
Low-level Mesocyclogenisis (Doswell)
- Numerical experiments show that low-level mesocyclogenisis occurs sooner, is stronger, and is more
persistent in simulations that have a boundary present. This is because the boundary now provides
the source of streamwise vorticity in the storms inflow instead of the forward-flank downdraft.
Therefore the development and strength of the storm-generated forward-flank cold pool is less
important to low-level vorticity generation, as the initial storm almost immediately begins to ingest
significant low-level streamwise vorticity from its mesoscale environment.
- near ground the mesocyclone may form due to the tilting of horizontal vorticity generated within
low-level density gradients. Barotropic processes may also play a role in low-level mesocyclogenisis.
Mid-level Mesocyclogenisis (Doswell)
- at mid-levels the mesocyclone develops initially from the tilting of horizontal vorticity associated with
the vertical shear of the environmental winds